The Senate took a rare step on Thursday and held a company in contempt of Congress -- Craigslist competitor Backpage.com.

Citing the First Amendment, Backpage has steadfastly refused to comply with a Senate-issued subpoena in a sex trafficking investigation.

It was the first time the Senate issued contempt charges in nearly 21 years. The last such action was taken in 1995, during a Senate investigation of Bill Clinton's controversial Whitewater properties. The Senate has only taken such action six times since Congress passed its civil enforcement statute in 1979, according to the Congressional Research Service.

The classifieds site has been accused of allowing customers to post advertisements for prostitutes, including children.

The Senate issued a subpoena to Backpage in October 2015. But Backpage and its CEO, Carl Ferrer, have refused to answer questions. The company says the First Amendment protects it from complying with the subpoena.

"The Senate Subcommittee believes that the First Amendment does not limit its activities with respect to Backpage.com," said Steve Ross, partner at Akin Gump and counsel to Backpage.com, in a statement. "The company, and its legal counsel, disagrees."

Led by Senators Rob Portman and Claire McCaskill, the Senate Permanent Subcommittee on Investigations has called Backpage "the most important player" in the commercial sex advertising market. The company's annual sales are more than $150 million, $100 million of which comes from adult advertising, according to the subcommittee.

Backpage claims to combat human trafficking, saying that it screens posts for illegal activities. But a subcommittee investigation found that Backpage helps to facilitate sex trafficking.

For example, the site edits ads before they're posted, often removing evidence of law-breaking in its "adult" classifieds, the probe determined. Backpage removes certain words from ads that would trip alarm bells, and it permanently deletes metadata from posts' images that could allow law enforcement to determine who posted an illegal ad, according to the Senate investigation.

The Senate probe also concluded that the site helped customers continue to receive payments for their posted services, even after MasterCard and Visa stopped processing payments for Backpage in July 2015. American Express stopped processing payments earlier in 2015.

The National Center for Missing and Exploited Children testified in a subcommittee hearing in November that 71% of all reports of suspected child sex trafficking have a link to Backpage.

The subcommittee voted 15-0 in February to hold the company in contempt of Congress. The whole Senate affirmed that vote unanimously on Thursday.

Now, the Senate Legal Counsel can opt to file a lawsuit against the company.

The company has previously stated that it would welcome a lawsuit, so it can defend its First Amendment stance in court.

"Backpage.com looks forward to a proper consideration of the important First Amendment constitutional issues by the judiciary -- the branch of government charged with protecting the constitutional rights of all Americans," said Ross, the lawyer for Backpage.